Compression ignition engines provide advantages in fuel economy, but produce both NO.sub.x and particulates during normal operation. When primary measures (actions that affect the combustion process itself, e.g., exhaust gas recirculation and engine timing adjustments) are taken to reduce one, the other is usually increased. Thus, combustion conditions selected to reduce pollution from particulates and obtain good fuel economy tend to increase NO.sub.x.
Current and proposed regulations challenge manufacturers to achieve good fuel economy and reduce particulates and NO.sub.x. Lean-burn engines will be necessary to achieve the fuel economy objective, but the high concentrations of oxygen in the exhaust renders typical exhaust gas catalyst systems ineffective for reducing NO.sub.x.
SCR (selective catalytic reduction) has been available for years in some contexts for reducing NO.sub.x. Originally, SCR depended on the use of ammonia gas, which has safety problems associated with its storage and transport. Aqueous urea and solid reagents are safer, but were not initially practical for many SCR applications--particularly mobile NO.sub.x sources--due to the difficulty in converting them from solid or solution form to active gaseous species, typically NH.sub.3 and HNCO radicals.
Because of the heightened awareness and concern with emissions, there was a need for a safe, economical and effective answer to the problems associated with SCR, particularly for mobile compression ignition engines. Where SCR catalysts had been employed to limit NO.sub.x emissions from compression ignition engines, one had to deal with either the dangers of ammonia leakage or using a urea solution or other reagent and risk fouling the catalysts under most conditions. In this regard, see R. J. Hulterman; A Selective Catalytic Reduction Of NO.sub.x from Compression ignition Engines Using Injection Of Urea; Ph.D. thesis, September 1995. Hulterman describes a number of technical challenges including clogging of atomizers, decomposition problems and system dynamics.
The first limited attempts to use urea SCR for compression ignition engines sometimes required the use of large pyrolization chambers or other devices following the point of urea introduction into the exhaust, as disclosed in U.S. Pat. No. 5,431,893, to Hug, et al. Equipment of this type highlights the known problems with urea.
Regardless of physical form, urea takes time to break down in hot exhaust gases and may cause nozzle plugging. To protect an SCR catalyst from fouling, Hug, et al., propose bulky equipment. In addition, that disclosure highlights the necessity of maintaining the urea solution at a temperature below 100.degree. C. to prevent hydrolysis. They propose the use of moderate urea solution pressures when feeding the urea solution and find it necessary to have alternative means to introduce high-pressure air into the feed line when it becomes plugged. The nozzles employed by Hug, et al., are apparently capable of producing moderately-fine sprays, the dispersion of which is aided by auxiliary air, but the droplets are still large enough to require a large pyrolization channel. Moreover, they employ dilute solutions that require significant heating to simply evaporate the water.
In European Patent Specification 615,777 A1, there is described an apparatus that feeds solid urea into a channel containing exhaust gases, which are said to be hydrolyzed in the presence of a catalyst. For successful operation the disclosure indicates that it is necessary to employ a hydrolysis catalyst, compressed air for dispersion of fine solids, means for grinding the urea into fine solids and a coating to prevent urea prills from sticking together. The disclosure notes that if the inside of the catalyzer and the nozzle tip only were coated with the catalyst, corrosion and deposition occurred. Despite achieving the goal of removing water from the process, the specification introduces solid urea into the gas stream-possibly depositing urea on the SCR catalyst.
Some of the deficiencies associated with these prior art systems were addressed by the device disclosed in U.S. Pat. No. 5,809,775. That device employs solid reagent in an SCR system integrated with an engine and provides an improved process and apparatus for NO.sub.x reduction. The device includes a vessel with a solid NO.sub.x -reducing reagent that generates ammonia gas when it is heated to a temperature above the pyrolysis temperature for the reagent. The device then introduces the ammonia gas into the exhaust gas at an exhaust gas temperature effective for selective catalytic reduction, and passes the exhaust gas containing the reactant gas through an SCR reactor.
As noted in the '775 patent, the temperature of the exhaust gas is preferably within the range of from about 180.degree. to about 650.degree. C. Urea is a preferred solid NO.sub.x -reducing reagent, but the reagent can comprise a member selected from the group consisting of: ammelide; ammeline; ammonium carbonate; ammonium bicarbonate; ammonium carbamate; ammonium cyanate; ammonium salts of inorganic acids, including sulfuric acid and phosphoric acid; ammonium salts of organic acids, including formic and acetic acid; biuret; cyanuric acid; isocyanic acid; melamine; tricyanourea; amines and their salts (especially their carbonates), including guanidine, guanidine carbonate, methyl amine carbonate, ethyl amine carbonate, dimethyl amine carbonate, hexamethylaminetetramine and hexamethylaminetetramine carbonate.
Each of the above NO.sub.x -reducing reagents, or combination of them, will have a preferred temperature for pyrolysis. Some like ammonium carbonate, ammonium bicarbonate, and ammonium carbamate, are converted easily with relatively mild heating, e.g., as low as 40.degree. C. The conversion of these materials is quantitative to ammonia gas and carbon dioxide. Others of the NO.sub.x -reducing reagents are "urea-related materials" (ammelide, ammeline, ammonium cyanate, biuret, cyanuric acid, isocyanic acid, melamine, tricyanourea, urea, and mixtures of any number of these) and do not decompose easily and yield HNCO in addition to ammonia as active reagent gases. Yet others, do not form HNCO, but decompose to a mixture of gases including hydrocarbons. Among this group are various amines and their salts (especially their carbonates), including guanidine, guanidine carbonate, methyl amine carbonate, ethyl amine carbonate, dimethyl amine carbonate, hexamethylaminetetramine and hexamethylaminetetramine carbonate. Amines with higher alkyls can be employed to the extent that the hydrocarbon components released do not interfere with the NO.sub.x -reduction reaction.
As further noted in the '775 patent, it is preferred that, for the group of urea and other urea-related materials, the solid NO.sub.x -reducing reagent be heated to a temperature of at least about 300.degree. C. and is maintained at a temperature at least about as high until introduced into the exhaust gas. Positive pressure or vacuum can be applied during heating the NO.sub.x -reducing reagent. Conveniently, the reagent is maintained under a pressure of at least about 50 psi during heating to assist with injection of the reactant gas.
While the prior art systems generally perform acceptably while there is NO.sub.x -reducing reagent present in the vessel, the prior art systems do not reduce NO.sub.x emissions when the vessel is empty or when the vessel is filled with a substance other than an acceptable NO.sub.x -reducing reagent. It would be preferable to have an SCR system that is integrated with an engine, that provides an improved process and apparatus for NO.sub.x reduction while better insuring that there is appropriate NO.sub.x -reducing reagent present in the vessel.